This invention relates to an improved sensor of the type having a ceramic element that undergoes a change in an electrical characteristic in response to a change in the partial pressure of oxygen in a mixture of gases to which the ceramic element is exposed. The ceramic element of the sensor may be either titania or zirconia under the current state of development, but other electrically responsive ceramics are known and may be used in the future. The preferred titania ceramic element is porous to provide a large surface area for effecting the transfer of oxygen from the titania to the gases to which the ceramic element is exposed and vice versa. According to the present invention, the porous titania ceramic element has dispersed within it a discontinuous coating of a precious metal charge transfer material. In application Ser. No. 839,701 mentioned above, this precious metal charge transfer material was referred to as a "catalyst", a somewhat misleading designation.
Sensors of the type having a zirconia ceramic element also utilize a porous platinum charge transfer material, but the zirconia ceramic is very dense and the platinum is applied to the zirconia surfaces by vapor deposition. The surface platinum to be exposed to engine exhaust gases is usually covered with a porous refractory material to aid in bonding and for the protection of the platinum.
Sensors of the type discussed above are particularly suited for use in detecting excursions, above and below stiochiometry, of the air/fuel ratio of the mixture of air and fuel supplied to an internal combustion engine. In accomplishing this detection, the sensor is positioned in the path of the exhaust gases emanating from the engine. As the mixture supplied to the engine changes from rich to lean, the exhaust gases change from a composition including very littly oxygen to a composition containing an excess of oxygen. As the exhaust gases change from lean to rich, the reverse changes in composition occur. The sensors have an electrical characteristic that undergoes a step-function change as a result of the mixture excursions across the stoichiometric air/fuel ratio.
The titania ceramic material undergoes a change in its resistance as a function of the oxygen concentration gradient between the titania and the exhaust gases. The zirconia ceramic element undergoes a change in the EMF produced across its platinum change transfer electrodes as a function of the oxygen concentration differential on opposite sides of the zirconia material. With the zirconia sensor, a reference gas, usually air, is applied to one side of the zirconia and the exhaust gas composition is allowed to contact the other side of the zirconia. The use of a reference gas is unnecessary in connection with titania sensors, and the entire titania ceramic element is immersed in the exhaust gases.
The present invention is particularly directed to a titania sensor. The response time of the prior art titania sensor, to excursions from stoichiometry of the air/fuel ratio of the combustion mixture being provided to the internal combustion engine, is long (slow response) at exhaust gas temperatures in the lower portion of the normal operating temperature range. The prior art titania sensor has a normal operating temperature range that extends from about 525.degree. C. to about 900.degree. C. This is a range substantially narrower than that of the 350.degree. C. to about 850.degree. C. range of a typical zirconia exhaust gas sensor, and sensor operability at lower temperatures is a necessity if separate sensor heating devices are to be avoided. Failure of the prior art titania sensor to operate at low temperatures is a very serious problem because it means that the feedback fuel control system associated with the sensor for controlling the mixture ratios supplied to an internal combustion engine cannot be operated unless and until the exhaust gases have heated the sensor sufficiently to maintain its temperature above that at which it is able to respond to air/fuel ratio variations. This may increase undesirable engine exhaust emissions and reduce fuel economy during engine warm-up conditions.
The aforementioned technique of heating the titania sensor to avoid the response-time lag requires that the sensor be maintained at a preselected, elevated operating temperature. While this approach has proved to be functional, the reliability of the device is decreased and its manufacturing cost and complexity are excessive.